1. Field of the Invention
The present invention relates to a low-melting alloy and a cream solder using a powder of the alloy, which is improved in strength after melting and solidification while maintaining workability (solderability) in spite of the low melting point thereof.
2. Description of the Prior Art
Upon surface mounting of semiconductor devices such as for printed wiring boards or integrated circuits, a technique for bonding individual constituent devices plays an extremely important role. In particular, improvement in the technique of micro-soldering (soldering for minute portion) is indispensable for enabling high density mounting by increasing the integration degree of LSI substrates.
Then, as a method of exactly mounting minute semiconductor parts at predetermined positions on a circuit substrate, a method of using cream solder has generally been known. That is, semiconductor parts are fixed by utilizing the viscosity and the surface tension of a cream solder deposited in a desired pattern on the circuit substrate by a printing method and, subsequently, the parts are heated, for example, by a hot air or infrared rays to solder the minute parts. Therefore, for the cream solder, demands for workability and reliability have been increased more and more with respect to low reflow temperature (low melting point), good wettability and bonding strength after cooling and solidification.
Generally,, the cream solder is prepared by mixing a powdery metal alloy with a highly viscous paste flux into a creamy state. As the metal powder, for satisfying various demands as described above, eutectic solders of Sn/Pb composition with addition of Bi, In, Cd, etc. have been known (refer, for example, to Japanese Patent Laid-Open Nos. Sho 47-6755, 63-112092 and 63-238994).
However, the conventional metal powder for cream solder involves a problem that two characteristics of low operating temperature and reliability for bonding strength are not always compatible. That is, the Sn/Pb eutectic solder described above is excellent in mechanical strength for the bonded portion, but involves a problem that the operating temperature is higher than 210.degree. C. and can not attain the aimed low soldering temperature.
Bi, In, Cd described above are added for lowering the melting point. The melting point can be controlled by adding such additive elements in a certain ratio to usual soldering alloys. However, among Bi, In and Cd, In is expensive and Cd is toxic, so that they involve a problem also in view of practical use.
The present invention has been accomplished taking notice of such prior art and provides a low-melting alloy having all the performances required as the low-melting alloy for cream solder and also excellent with view points of cost and safety, as well as a cream solder using a powder of the alloy.